1. Field of the Invention
The present invention relates to a process for producing an oxymethylene copolymer excellent in heat resistance and mechanical properties. It also relates to an oxymethylene copolymer composition.
2. Description of the Prior Art
It has been known from, for example, Japanese patent Publication No. 14640/61 that oxymethylene copolymer is obtained by polymerizing trioxane and cyclic ether in the presence of a catalyst. Since the polymer thus obtained is thermally unstable, terminal sealing through esterification, or degradation and removal of unstable chain ends, is carried out for stabilization, before which, however, deactivation of the catalyst in order to inhibit further reaction is required. That is, polyoxymethylene derived from cationoid polymerization of trioxane or the like will slowly generate depolymerization, resulting in remarkably lowered molecular weight and/or extreme thermal instability, unless the catalyst remaining therein is deactivated.
As to deactivation of a boron trifluoride polymerization catalyst, U.S. Pat. No. 2,989,509 discloses the use of aliphatic amine or heterocyclic amine. By deactivating the catalyst with such amine followed by washing to remove them, the polymer is stabilized enough to avoid lowering the molecular weight when preserved as it is for a long period.
However, unless the catalyst is removed from the polymer by washing after it is deactivated by a general amine compound, depolymerization will still occur and the molecular weight will be lowered when the polymer is melted or dissolved. Accordingly, it has been essential to remove the catalyst from the polymer by a thorough washing operation after the catalyst is deactivated by normal amine. Japanese Patent Publication Nos. 8071/64 and 1875/68, for example, propose methods of stabilizing copolymer chain ends, in which cases also prior to starting the chain ends stabilizing process, a polymerization catalyst is deactivated by amine and then removed by washing. In order to produce an oxymethylene copolymer with higher thermal stability, however, it is essential to blend a stabilizer as well as to degradate the instable chain ends after deactivation of the catalyst. For this purpose, a variety of stabilizers have conventionally been known such as polyamide and hindered phenol to be milled for stabilization according to Japanese patent Publication No. 5440/59.
By milling such stabilizers as mentioned above, the thermal stability is surely increased, but not satisfactorily, because when the polymer is kept in the melted state for a long time in a molding process or the like, decomposition with foaming occurs and mold-deposits are generated. In addition, if washing to remove the catalyst is omitted in the process of catalyst deactivation by normal amine, thermal stability of the polymer will remain too low to withstand molding or the like, whichever stabilizer is milled.